Known in the art are white coloured enamels (cf., e.g., UK Pat. Nos. 948,270 and 795,464) based on the use of acrylic resins and other vehicles and containing as pigments anatase or rutile titanium dioxide, zinc oxide, lead sulphate, lead carbonate, magnesium oxide, and other compounds.
Dark coloured heat-reflective coatings are essential in some applications, thus as deck coatings for oil carrier tankers where light coloured coatings may adversely affect in bright sunlight the working conditions and performance of deck hands.
A need exists in the art therefore for the development of dark coloured heat-reflective enamels.
One commonly known heat reflectance formulation is a green coloured pentalkyd enamel used as a deck coating on oil carrier tankers.
This heat-reflective pentalkyd enamel is an alkyd resin solution with lemon chrome, iron blue and iron oxide red pigments dispersed in it.
Said heat-reflective enamel, however, has poor heat reflectance, insufficient weather resistance, and a short coating life, consistent with the formulation.
Taken together these factors explain high oil product losses and the need for frequent recoating where said enamel is used on storage and transportation tanks for oil products or on oil carrier decks.
Product losses due to evaporation can be reduced by minimizing heat build-up in the coated surfaces. An indirect method for assessing the heat reflectance properties of a coating under laboratory conditions is provided by measuring the temperature of the reverse side of a coated metal specimen irradiated by means of an incandescent lamp.
For the above pentalkyd enamel this temperature runs as high as 75.degree. C., which is indicative of its low heat reflectance characteristics.
For lower heat build-up due to solar radiation in surfaces to be protected, use should be made of enamels that are highly reflective in the visible, and more particularly infrared, spectral regions.
Currently, a dark coloured heat-reflective enamel is known in the art (cf., e.g., U.S. Pat. No. 3,577,379, U.S. Cl. 260-40), incorporating a weather resistant vehicle, pigments, extenders, and a solvent.
Said enamel is obtained by dispersion in a resin solution of a pigment mixture, extenders, driers such as cobalt naphthenate, and various additives such as stabilizers and antioxidants. The pigment mixture combines pigments that are necessary to provide dark colour to the enamel, such as lead chromate, molybdate organe, phthalocyanine blue, etc., with pigments of high infrared reflectance (spectral region of 0.2 to 2.5 microns), such as quinacridone red.
The extenders used in said heat-reflective enamel are the siliceous matter or mixture of siliceous matter and barytes.
The vehicles used are styrenated alkyd resins, chlorinated paraffins and like.
Said heat-reflective enamel is typically formulated as follows, in parts by weight:
Lead chromate: 56.4 PA0 Quinacridone red: 6.5 PA0 Molybdate orange: 25.2 PA0 Phthalocyanine blue: 4.4 PA0 Titanium dioxide: 7.4 PA0 Magnesium silicate: 122.2 PA0 Styrenated alkyd resin: 130.0 PA0 Xylene: 325.0 PA0 Diethylamine: 0.3 PA0 Cobalt naphthenate: 0.6 PA0 Antioxidant: 0.6 PA0 titanium dioxide or metatitanic acid based on titanium dioxide: 100 PA0 nickel compound thermally decomposed to yield nickel oxide, based on nickel oxide: 3 to 32 PA0 antimony trioxide: 15 to 35 PA0 ammonium dichromate or an alkali metal dichromate, based on chromium oxide: 2 to 5 PA0 vehicle: 100 PA0 pigment: 30 to 80 PA0 extender: 1 to 60 PA0 solvent: 90 to 840 PA0 product of thermal interaction between titanium dioxide or metatitanic acid, nickel compound thermally decomposed to yield nickel oxide, antimony trioxide, and ammonium dichromate or an alkali metal dichromate: 15 to 60
The formulation is seen to include diethylamine as stabilizer and cobalt naphthenate as drier. The antioxidant can be any conventional type.
The heat reflectance properties of the aforesaid heat-reflective enamel are not sufficiently high, as evidenced by the reverse side temperature of the coated specimen, which is equal to 72.degree. C. Considerable oil product losses are observed at such a level of temperature.
The coating life is not sufficiently long, being equal to nine months.
Furthermore, the formulation contains quinacridone red, the production technology for this pigment being rather complicated, with the process multistage and a large number of by-products formed.
Considering the disadvantages outlined, the heat reflectance enamel as formulated above is not suitable for use as a deck coating for oil product carrier tankers.